Localization of substance P(SP) receptor was conducted on the midbrain of adult Sprague-Dawley (SD) and spontaneously hypertensive rats (SHR). Significant difference of 79.9 and 32.2% lesser in density of the receptor distribution was observed in zona inserta and lateral hypothalamus respectively in SHR than in Sprague Dawley. The chemical and physical analysis of the receptor proteins isolated and purified form olfactory bulb and intestine reveals differences in the isoelectric points and in binding displacement with SP antibodies. Further investigation is underway to examine both chemical and functional differences in the different SP receptors types. SP and SP receptors were found densely and uniformly distributed in uterus of Sprague-Dawley rats. The receptors appeared to migrate during pregnancy toward the basal endometrium. The specific binding/mg wet tissue decreased as the pregnancy progressed (2,200 cpm/mg in nonpregnant uterus to 1,000/mg of uterus at full term). The disproportional growth in the uterus and its SP receptors would be of benefit in decreasing pain perception during pregnancy. SP was found densely distributed in atrial muscle but few SP receptors were observed under the conditions examined.